(a) Field of the Invention
The present invention relates to an air valve assembly for hydraulic jack, and more particularly to such an air valve assembly, which can be easily installed in the oil cylinder of a hydraulic jack keep the internal pressure of the hydraulic jack in balance with the atmospheric pressure, and to prevent hydraulic oil from leaking out.
(b) Description of the Prior Art
The oil cylinder structure (see FIG. 2) of a conventional hydraulic jack (see FIG. 1) is generally comprised of an outer oil cylinder 10, an inner oil cylinder 20, a movable rod member 30, and a valve block 40. The inner oil cylinder 20 is fixedly mounted inside the outer oil cylinder 10. The rod member 30 is installed in the inner oil cylinder 20, and partially extended out of the front side of the outer oil cylinder 10. The valve block 40 is fixedly fastened to the outer oil cylinder 10 and the inner oil cylinder 20 at the rear side. Further, hydraulic oil is filled in the outer oil cylinder 10. When in use, the valve block 40 is operated to cause a suction force, which sucks hydraulic oil from the outer oil cylinder 10 into the inner oil cylinder 20. When hydraulic oil is delivered from the outer oil cylinder 10 to the inner oil cylinder 20, the rod member 30 is moved forwards to lift the load. In order to let hydraulic oil pass between the outer oil cylinder 10 and the inner oil cylinder 20, an air hole 101 is provided at the peripheral wall of the outer oil cylinder 10 for enabling outside air to pass into the outer oil cylinder 10 so that hydraulic oil can be completely drawn out of the outer oil cylinder 10 into the inner oil cylinder 20. However, hydraulic oil may leak out of the air hole 101 when returning to the outer oil cylinder 10. In order to eliminate this problem, a plug 102 or manual air valve may be used to seal the air hole 101. However, this structure of plug 102 cannot absolutely eliminate oil leakage. When a manual air valve is used, it must be opened upon each use of the hydraulic jack, and then closed after each use of the hydraulic jack. There is another conventional design, which provides a relatively bigger volume to hold hydraulic oil and air, preventing insufficient hydraulic oil suction action due to insufficient pressure difference at the end of the stroke. This design is an enclosed design. The main drawback of this design is its bulky size.